Method and apparatus for mold temperature control

ABSTRACT

A method and apparatus (10) is disclosed which provides automatic control of the temperature of a fluid cooled plastic mold (12) during successive injections of liquid plastic into the mold. The temperature of the mold (12) is sensed by a probe (18) which provides a signal indicative of the temperature of the mold. The signal from the probe (12) is compared by the apparatus with a selected control temperature signal indicative of the proper operating temperature of the mold. When the sensed temperature is above the control temperature, a solenoid operated control valve (50) is activated so that cooling fluid, such as ordinary tap water, is passed through cooling channels (13) in the mold. Flow of cooling fluid is shut off when the sensed temperature drops below the selected operating temperature. The apparatus (10) also includes an undertemperature signal light (54) which lights to inform the operator that the mold is below a minimum operating temperature; and an overtemperature alarm light (46) and a speaker (44) which are both activated to warn the operator when an unacceptable overtemperature in the mold is sensed.

TECHNICAL FIELD

The present invention relates to molds for casting thermoplastics ingeneral, and, in particular, to a device for controlling the temperatureof such a mold while it is in operation.

BACKGROUND ART

The prior art is generally cognizant of the need to disperse excess heatfrom molds used in the formation of molded, thermoplastic objects.Molten thermoplastic material is introduced into a mold and allowed toset or cure by cooling. Generally, the mold is cooler than the moltenplastic and absorbs the heat that must be removed from the moltenthermoplastic before it will set. Consequently, as a series of objectsare molded successively, the temperature of the mold tends to increase.

It is generally desirable to maintain the temperature of the mold at aparticular level found to be that which allows a plastic object beingmolded to exhibit the least possible amount of shrinkage and distortionduring the setting or curing process. In any event, it is important tomaintain the mold at a consistent operating temperature so as to providefor uniformity among the replications of the object being molded. It isalso desirable to remove excess heat from the mold promptly after themolding of an object so as to more quickly prepare the mold for asubsequent introduction of molten plastic, thus increasing theefficiency of production possible by use of the mold.

The control of the temperature of a mold by circulating fluids throughchannels fashioned in the walls of the mold is known in the art.Conventionally, the fluid is heated to the desired ideal operatingtemperature and then is circulated through the mold before the firstinjection or "shot" of hot plastic is introduced. The temperature of themold increases upon the introduction of molten plastic but is restoredto the operating temperature by the continued circulation ofconsiderable quantities of fluid, the temperature of which is maintainedat the ideal operating temperature. The fluid is required to circulatesubstantially all the time that the mold is being used in makingsuccessive replications of the object molded.

DISCLOSURE OF THE INVENTION

The mold temperature controller for a fluid-cooled mold in accordancewith the present invention includes means for sensing and conveyinginformation about the temperature of the mold, means for responding tothe information so conveyed by selecting and sending an appropriatecontrol signal when the temperature sensed exceeds a selected maximumlevel, and a signal responsive fluid regulator adapted to be connectedin the flow of cooling fluid to the mold, the regulator opening andclosing in response to the control signal to regulate the flow ofcooling fluid to the mold. A method for controlling the temperature of afluid-cooled mold includes the steps of injecting liquid plastic intothe mold, sensing the temperature of the mold, and responding to thesensed temperature by passing cooling fluid through the mold when thesensed temperature is above a selected control temperature and cuttingoff the flow of cooling fluid below the control temperature. In thismanner the mold can be heated to and maintained at the desired workingtemperature using a cooling fluid the temperature of which need not beelevated to or maintained at the ideal working temperature of the mold.

A second object of the invention is to provide for the cooling of afluid-cooled mold without the need for a continuous flow of fluid,thereby reducing the consumption of fluid and allowing ordinary tapwater to be intermittantly passed through the mold to cool it.

In addition, the mold is elevated in temperature by heat from moltenplastic within the mold, so that no additional heaters are required;thus, energy consumption is substantially lower than in conventionalsystems which heat the circulating fluid and pump the fluid through themold.

Other objects, features, and advantages of the invention will beapparent from the following detailed description taken in conjunctionwith the accompanying drawings showing a preferred embodiment of anapparatus for mold temperature control exemplifying the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus for mold temperaturecontrol constructed in accordance with the present invention, shown inassociation with a mold.

FIG. 2 is a cross-sectional view of the mold of FIG. 1 taken alongsection lines 2--2 of FIG. 1.

FIG. 3 is a schematic block diagram showing the fundamental functionalelements of the mold temperature controller of FIG. 1 and theirrelationship to one another.

FIG. 4 is a schematic circuit diagram of a portion of the electricalcircuitry of the mold temperature controller of FIG. 1.

FIG. 5 is an illustrative graph showing typical temperature changes in amold during successive injections of molten plastic.

FIG. 6 is a cross-sectional view taken along the longitudinal centerlineof an alternative embodiment of a temperature probe shown illustrativelyplaced in a mold.

FIG. 7 is a cross-sectional view taken along the longitudinal centerlineof a second alternative embodiment of a temperature probe shown placedon a mold.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring more particularly to the drawings, wherein like numbers referto like parts, FIG. 1 shows an apparatus for mold temperature control,generally indicated at 10, constructed in accordance with the presentinvention.

The mold temperature controller 10 is used in conjunction with afluid-cooled mold, shown generally at 12 in an illustrativeconstruction, although it is apparent that many different moldconfigurations may be utilized. As shown in the cross-sectional view ofFIG. 2, the mold 12 has internal surfaces defining cooling channels 13which pass through the mold 12 in locations selected to allow fluidflowing through the channels to efficiently absorb heat from the mold. Asource of cooling fluid, such as the water pipe 14, is connected withthe channels 13 by any convenient means. The cooling liquid may beexhausted or recycled through a cooling tower to dissipate the heatabsorbed from the mold, or, if ordinary tap water is used, the waterpassed through the mold may simply be drained to a sewer system. Themold 12 also has internal surfaces defining a mold cavity 15 adapted toreceive an injection or "shot" of molten thermoplastic 16 to be cooledand thereby "set" or "cured" to form the object to be molded.

The mold temperature controller 10 has a temperature sensor 17 toprovide a means for sensing and conveying information about thetemperature of the mold 12. In the embodiment shown in FIG. 2, thetemperature sensor 17 includes a probe 18 having a resilient shank 20containing electrical leads and a contact head 22 formed of a heat andabrasion resistant material such as epoxy located at an end of the shankwhich houses a thermistor 24 (not shown in FIG. 2). An insulatedelectrical lead 26, a portion of which may serve as the shank 20,extends from the thermistor 24 back to a case 27 which houses theelectrical components of the controller.

Internal surfaces in the mold 12 define a probe well 28 which extendsinwardly from an external surface 30 toward the cavity 15 for a selecteddistance such that the bottom of the probe well is separated from thecavity by a relatively thin wall of metal (e.g., 4 mm). The probe well28 is adapted to receive the shank 20 of the temperature probe 18 withthe contact head 22 touching the bottom of the probe well. Thetemperature probe 18 is secured in the probe well 30 such that thecontact head 22 of the probe remains in firm contact with the bottom ofthe probe well 28 with the shank slightly bent. In the preferredembodiment, the probe 18 is secured in the well by a frustum shapedsleeve 32, made of a compressible and resilient material, such as rubberor a resilient plastic, which surrounds a portion of the shank 20 and isadapted to frictionally engage the walls of the well 28.

Preferably, the probe well 28 extends to a point adjacent to that partof the mold 12 which becomes hottest when the mold is injected withmolten plastic--usually an injection sprue or other part of the cavity15 near to the point at which the shot of molten plastic is injectedinto the cavity. For example, the contact head 22 of the temperatureprobe 18 will preferably be located within approximately 2.5 mm of thehottest accessible area of the cavity 15. By locating the probe in thismanner, the thermistor 24 can provide a signal that varies with thetemperature of the hottest part of the mold 12.

The mold temperature controller 10 includes means for responding to theinformation about the temperature of the mold 12 provided by the sensor17 and means for generating an appropriate control signal when thetemperature sensed exceeds a selected maximum. In the preferredembodiment, the signal from the temperature sensor 17 is conveyed tothree comparators, as illustrated by reference to the block diagram viewof FIG. 3. A first alarm comparator 34 provides an alarm signal S_(a) ifthe information conveyed from the temperature sensor indicates that themold 12 is at a temperature T in excess of a selected alarm temperaturelevel T_(a) provided by an alarm temperature selector 35. A secondcontrol comparator 36 provides a control signal S_(c) if the temperaturesensor indicates that the temperature of the mold 12 exceeds a selectedcontrol temperature level T_(c) provided by a control temperatureselector 37. A third undertemperature comparator 38 provides an undertemperature signal S_(u) if the temperature indicated by the temperaturesensor is less than a selected minimum operating temperature level T_(u)provided by an under temperature selector 39.

In the event that the first comparator 34 provides an alarm signalS_(a), the alarm signal is received by and triggers an oscillator 40.The output of the oscillator 40 is received by an alarm driver 42 thatdrives a speaker 44 to emit an audible alarm. The output from theoscillator 40 is also received by an alarm light 46 which isconveniently located so as to be observable by the operator of the mold12.

If the temperature T is such that the second comparator 36 provides acontrol signal S_(c), the control signal is received by a valve driver48 that provides power to open a cooling fluid valve 50 and therebyrelease a flow of cooling fluid to the mold 12. In the preferredembodiment, the control signal S_(c) provided by the second comparator36 is also received by a cooling fluid on light 52, mounted on the faceof the controller case 27 where it may be observed conveniently by theoperator.

If the temperature is below the minimum temperature T_(u), such that thethird comparator 38 provides a low temperature warning signal S_(u), thewarning signal is provided to an under temperature light 54. The undertemperature signal S_(u) and the over temperature signal S_(a) areprovided to a logic circuit 56 which provides an output signal to drivea normal temperature light 58 when the signals S_(a) and S_(u) are bothabsent or "low." If either of the signals S_(a) or S_(u) are present or"high," the normal temperature light 58 will be off. The presence ofthis light thus provides a positive indication to the operator that thecontroller is working properly.

When an apparatus for mold temperature control constructed in accordancewith the present invention is used to control the temperature of afluid-cooled mold, the following events and steps generally occur.Before the first injection of molten thermoplastic, the mold 12 istypically cooler than the desired operating temperature. With eachinjection of plastic, the temperature of the mold increases. Usuallythree or four injections of plastic are sufficient to heat the mold tothe desired operating temperature, the objects molded as a consequenceof these start-up injections being discarded. In the representativegraphical plot of mold temperature against time shown in FIG. 5, thetemperature of the mold may be seen to increase with each injection ofplastic, taken to occur at points 62 on the graph. A temperature T_(u)is selected that is at the minimum desired operating temperature for themold 12, and the third comparator 38 is adjusted to provide a signalS_(u) so long as the temperature T of the mold 12 remains below T_(u).Consequently, as set forth above, the under temperature light 54 remainson until T equals or exceeds T_(u). Similarly, the normal temperaturelight 58 remains off so long as T is less than T_(u).

At some point as molten plastic is repeatedly injected into the mold 12,the measured temperature T of the mold 12 will exceed the desiredoperating temperature. A temperature T_(c) is selected being either thesame or slightly higher than the preferred operating temperature. Thesecond comparator 36 is adjusted to provide a control signal S_(c) whenthe temperature of the mold T exceeds T_(c). As explained above, thesignal S_(c) causes the valve driver 48 to open the cooling fluid valve50 and cause cooling fluid to pass through the cooling channels 13. Whenthe temperature of the mold 12 has been reduced beneath the controltemperature T_(c), the second comparator 36 ceases to provide thecontrol signal S_(c), and the flow of cooling fluid is turned off. Thecooling fluid on light 52, being activated by the control signal S_(c),remains on so long as the cooling fluid is passing through the mold.

With each successive injection of molten plastic, the temperature of themold 12 usually is observed to rise above the control temperature T_(c),causing the second comparator 36 to activate the valve driver 48 andcooling fluid on light 52, in the manner described above. A typicalpattern of temperature change during such a series of molten plasticinjections is shown in FIG. 5 at 64. The temperature T rises above thecontrol temperature T_(c) for a brief period of time until the flow ofcooling fluid reduces the temperature to the control temperature onceagain. Because the cooling fluid may be selected to be considerablycooler than the desired operating temperature, the time necessary torestore the mold 12 to the operating temperature after each injection ofmolten plastic may be relatively brief, allowing for the efficient useof the mold, which is ready for the next injection of plastic as soon asthe last has been set and ejected from the mold. It will be apparentthat the use of cooling fluid having a temperature considerably belowthe desired operating temperature allows for a swifter cooling of themold 12 than could be accomplished with a comparable flow of coolingfluid maintained at the operating temperature, as is conventionallydone. If, for some reason, it is desired to slow the cooling of the mold12, means for limiting the flow rate of cooling fluid may be employed,such as the adjustable valve 66. The valve 66 is adapted to close thewater pipe 15 to a selected extent.

Should the flow of cooling fluid be insufficient to cool the mold 12,the temperature of the mold will rise with each succeeding injection ofplastic, as indicated at 68 in FIG. 5. A failure of cooling fluid flow,selection of too slow a flow rate, or too frequent injections of plasticmay all lead to this result. Should the temperature of the mold 12exceed a selected alarm temperature T_(a), the first comparator 34 emitsan alarm signal S_(a) as described above, activating the oscillator 40and, in turn, the alarm light 46 and the speaker drive 42. The visualalarm from the alarm light 46 and the audible alarm from the speaker 44warn the operator to take appropriate action to reduce the temperatureof the mold to the desired operating temperature.

It is preferred that the control temperature level T_(c) provided to thesecond comparator 36 be adjustable to accommodate differing mold rates,mold geometries, plastic materials, and so forth. In the embodiment ofthe controller shown in FIG. 1, a control knob 70 mounted on the case ofthe controller is connected to vary the temperature level T_(c) at whichthe comparator 36 switches. Indicia on the case allow the controltemperature T_(c) to be correlated to various positions of the knob. Ina preferred embodiment of the controller, the under temperature levelT_(u) is automatically set at a selected number of degrees beneath thecontrol temperature T_(c) and the alarm temperature T_(a) likewisevaries automatically with T_(c). A separate temperature control knob 71is preferably provided whereby the amount by which the alarm temperatureT_(a) exceeds the control temperature T_(c) and the amount by which theunder temperature T_(u) is lower than the control temperature may beadjusted to reflect the operating tolerances desired.

An alternative embodiment of a temperature probe suitable for use withthe controller of the invention is shown generally in FIG. 6 at 72. Aninsulated lead 73 is contained within and extends from one end of acylindrical tube 74 that extends for substantially the length of thespring-loaded probe 72. A contact head 75 is located at the end of theinsulated lead 73, and contains a thermistor (not shown in FIG. 6). Thetube 74 has a first flared retaining ring 78 formed on its end adjacentto the contact head 75 and a second retaining ring 80 extends annularlyoutwardly from the other end of the tube 74.

A hub 82 has internal surfaces defining a cylindrical slide passage 84sized to slideably engage the tube 74 and a cylindrical spring well 86co-axial with and having a larger diameter than the slide passage. Aradially extending surface extends between the slide passage 84 and thespring well 86 to form a spring seat 87. A helical spring 88 surroundsand extends co-axially with the containing tube 74 and is received bythe spring well 86. One end of the spring 88 presses against the springseat 87, and the other end of the spring presses against the firstretaining ring 78, thus biasing the contact head 75 away from the hub82. The containing tube 74 is retained within the slide passage 84 bythe second retaining ring 80.

When adapted for use with the spring-loaded probe 70, the walls 90 ofthe probe well 30 are threaded, as shown in FIG. 5. Exterior threads 92on the hub 82 are adapted to threadedly engage the threads 90 of theprobe well 30, securely fastening the spring-loaded probe 70 in place.The containing tube 74 is of a length selected to reach the bottom ofthe probe well 30, and the spring 88 biases the contact head 75 firmlyagainst the bottom.

A third embodiment of a temperature probe is shown generally at 94 inFIG. 7. The magnetic probe 94 has a casing 96 made of magnetizedferromagnetic material having a disc-shaped back 98 and a circular sidewall 100 depending for a selected distance from the periphery of theback 98. The casing 96 has a port 99 and an insulated electrical lead102 which extends through the lead port and into the interior of thecasing 96. The lead 102 terminates in a contact head 104, facing awayfrom the back 98, which contains a thermistor (not shown). Fillermaterial 108, such as epoxy, fills the casing 96 and secures the lead102 and contact head 104 within the casing. The temperature probe 94 canbe applied to any convenient external surface of a mold and, if the moldis made of ferromagnetic materials, will be held in place by themagnetized casing 96.

The probe 94 is useful for monitoring the temperature of molds where thewalls are fairly thin or where it is not desirable to drill a hole intothe mold. Although the contact point and thermistor may not be in closeproximity to the mold cavity, the cooling temperature T_(c) can beadjusted to initiate cooling at a lower sensed temperature than thatexisting at the walls of the mold cavity.

The comparators 34, 36, and 38, oscillator 40, speaker driver 42, valvedriver 48, and logic circuit 56 may be implemented with conventionalelectronic circuitry and an exemplary preferred embodiment of suchcircuitry is shown in the schematic circuit diagram of FIG. 4. Anysuitable, conventional speaker and lights may be used for the speaker 44and the alarm light 46, cooling fluid on light 52, under temperaturelight 54, and normal temperature light 58. In the preferred embodiment,a conventional, solenoid activated fluid valve is employed as thecooling fluid valve 50.

An illustrative embodiment of the electronic circuitry of the controllerportion of the apparatus is shown in schematic form in FIG. 4. RegulatedDC power is supplied to the circuit from standard power supply circuitry(not shown) to power take-off points illustrated schematically at 110 inFIG. 4. Unregulated rectified DC power is supplied to the power take-offpoint 111 and is used to drive the speaker 44 and the solenoid operatedvalve 50. The temperature of the mold is sensed by the thermistor 24which forms part of a voltage divider with a second resistor 112, withthe thermistor varying in resistance with the temperature of the mold.As a result, the voltage on the line 114 between the thermistor 24 andthe resistor 112 will vary in relation to the temperature of the mold.This voltage is passed to the first comparator 36 which consists of adifferential operational amplifier 116 with associated biasing resistors117-119 and a capacitor 120. The voltage on the line 114 is fed to thenegative differential input of the operational amplifier 116 through aninput resistor 121. The output of the amplifier 116 is passed through adiode 123, a feedback resistor 124, a connecting line 125 and a secondinput resistor 126 to the positive differential input of the amplifier.An adjustable reference voltage input is provided by tapping off apotentiometer 130 connected between variable resistors 131 and 132, allof which are connected between the reference voltage and ground. Theresistance of the three resistors 130, 131 and 132 is varied by turningthe dial 70 on the face of the unit. Adjustment of these resistorschanges the temperature level T_(c) at which the comparator 36 willswitch states.

The output of the potentiometer 130 is also presented to the firstcomparator 34 at an input resistor 134 connected to the positivefollowing input of a differential operational amplifier 135, which isbiased by resistors 136, 137, and 138 and a capacitor 139. The output ofthe operational amplifier 135 is also fed back through a diode 140 andthrough the resistor 124 to the line 125 and thence to the inputresistor 134. The voltage from the thermistor 24 is provided on aconducting line 142 through a variable resistor 143 and a fixed resistor144 to an input line 145 leading to a resistor 146 which is connected tothe negative input of the operational amplifier 135. A compensationcapacitor 147 is connected between the negative input of the amplifierand the conducting line 125. An offset voltage of constant value is alsopresented on the line 145 from the voltage divider formed between aresistor 149 connected to the regulated DC power source and through theresistors 143 and 144 to the conducting line 142 and thence throughfixed resistors 150 and 151 and a variable resistor 152 to ground. Thevariable resistors 143 and 152 are ganged together and are operated bythe alarm temperature knob 71 on the face of the unit. Variation ofthese resistors thus provides a means to change the spread of degrees oftemperature between the normal operating temperature of the device andthe temperature T_(a) at which the over temperature alarm is triggeredor the temperature T_(u) at which the under temperature indicator isactivated.

The variation in resistance of the thermistor 24 appears as a variationin voltage on a conducting line 154 between the resistors 151 and 152,and this voltage is passed through an input resistor 155 to the positiveinput of an operational amplifier 156 within the third comparatorcircuit 38. The amplifier 156 has biasing resistors 157, 158, and 159and a biasing capacitor 160. The reference voltage on the line 125 isprovided to the negative input of the amplifier 156 through an inputresistor 161. The output of this amplifier is directed on a line 162through a diode 163 and thence back through the feedback resistor 124and the line 125 to the input resistor 161.

It will be seen that the feedback of the output signals from all threeof the operational amplifiers 116, 135 and 156, through the respectivediodes 123, 140 and 163 and the feedback resistor 124, providesstabilizing feedback for each of these amplifiers so that the amplifiersdo not saturate when they are turned on. When the negative input of eachof the differential amplifiers exceeds the positive input, eachamplifier attempts to draw down its output and will essential stabilizeat a zero output voltage level very quickly.

The output of the operational amplifier 116 and the comparator 36 isprovided to the valve driver circuit 48. This circuit includes a firsttransistor 170 which receives the output of the amplifier 116 through abase resistor 171 and which has the emitter thereof connected through acoupling resistor 172 to the base of a second power transistor 173. Abiasing resistor 174 is also connected between the base of the powertransistor 173 and ground. The collector of the first transistor 170 isconnected directly to the power source 111 whereas the power transistor173 has the solenoid coil 175 of the solenoid operated cooling fluidvalve 50 connected between the power source and its collector. A freewheeling diode 176 is connected across the solenoid 175 to conductinductive transient current from the solenoid during the timeimmediately after the transistor 173 is turned off. The solenoid 175will not be activated as long as the negative input provided through theresistor 121 to the amplifier 116 is greater than the correspondingreference voltage provided through the resistor 126 to the amplifier,during which time the output of the amplifier is low and the transistors170 and 173 are turned off. A decrease in the resistance of thethermistor 24 with increasing temperature will, at a selected point,cause the operational amplifier 116 to be switched to its stablized highlevel which will drive the transistors 170 and 173 on and pass currentthrough the solenoid 175. A light emitting diode 52 is connected to theoutput of the amplifier 116 through a resistor 177 and lights when thecontrol valve is on to so inform the operator.

The output of the under temperature comparator 38, from the differentialamplifier 156, is also passed through a resistor 180 to a light-emittingdiode which serves as the under temperature light 54.

The outputs of the comparators 34 and 38 are also passed to the logiccircuit 56 which performs a NOR function on the signals. The particularlogic circuit illustrated employs diodes 182 and 184 transmitting theoutputs of the amplifiers 156 and 135, respectively, to a common nodeconnected through a resistor 185 to the base of a PNP transistor 186.This transistor is turned off as long as either one of the outputs ofthe amplifiers 135 or 136 is high. If the outputs of both of theseamplifiers are low--indicating that the mold is operating within thenormal temperature range--then the base current flowing through abiasing resistor 187 is sufficient to turn the transistor 186 on andpass current through the light-emitting diode which acts as the normaltemperature indicator light 58. A by-pass resistor 188 and a seriesresistor 189 are provided to properly control the current passingthrough the indicator light 58.

When an alarm condition is sensed, the amplifier 135 will put out a"high" signal on its output line 190. The particular circuit shown inFIG. 4 provides an alternating frequency warning signal through thespeaker 44 which is more likely to alert the operator to the alarmcondition than a steady tone. The example of such a circuit shown inFIG. 4 utilizes currently available integrated circuit packages, withone suitable type being an MC 14011 quad NAND gate package. The signalon the line 190 is supplied to the power supply terminals 192 of thisintegrated circuit package to enable the devices thereon only during thetime that the amplifier 135 is providing an alarm output. This activatesan astable multivibrator formed of two NAND gates 194 and 195, afeedback capacitor 196 and a charging resistor 197. The relative sizesof the capacitor 196 and the resistor 197 are chosen such that theoscillating signal from the astable has a relatively long period, in therange of one second. The output of the first NAND gate 194 is deliveredthrough a coupling resistor 198 to a transistor 199 which, when turnedon, supplys current through a resistor 200 to a light-emitting diodeserving as the alarm light 46. This alarm light will thus flash on andoff approximately at one second intervals during the alarm condition.

The output of the second NAND gate 195 is passed through a diode 202 anda resistor 203 to a common node 204 which is connected to the input of asecond astable multivibrator. This second multivibrator includes twoadditional NAND gates 206 and 207, a feedback capacitor 208, andcharging resistors 209 and 210. The NAND gates 206 and 207 are againenabled only when a high signal is provided on the line 190, and, inaddition, the signal on the line 190 is provided through a connectingline 211 to one of the two inputs of the NAND gate 207, thus insuringthat an oscillating output is provided from this NAND gate only duringthe alarm condition. The frequency of oscillation will be determined bythe relative sizes of the resistors 203, 209 and 210 and the capacitor208. The change in the oscillation frequency occurs when the output ofthe NAND gate 195 switches: when the output of this gate is high, theoutput voltage is blocked by the diode 202 and the resistor 203 isessentially removed from the circuit; thus, the time constant of thecharging rate of the capacitor 208 will be determined entirely by thevalues of the resistances of the resistors 209 and 210; conversely, whenthe output of the NAND gate 195 is low, the current flowing through theresistor 209 will flow through the resistor 203 and the diode 202,thereby reducing the rate at which the capacitor 208 charges, and thuslowering the frequency of oscillation. A compensation capacitor 212 maybe connected in the circuit as shown between the common node 204 andground to improve the switching characteristics.

The oscillating output of the NAND gate 207 is provided to the base of afirst transistor 215 within the speaker driver 42. The emitter of thistransistor is connected through biasing resistors 216 and 217 to a powertransistor 220 which has the speaker 44 connected between the source ofunregulated DC power 111 and the collector of the transistor 220. Thefrequency of oscillation of the oscillator formed by the two NAND gates206 and 207 is chosen to be in the audio frequency range, e.g. 1,000 Hz,with the two frequencies of oscillation being perhaps an octave apart toallow them to be readily distinguished by the operator.

It is understood that the invention is not confined to the particularconstruction, materials, and arrangement of parts herein illustrated anddescribed, but embraces all such modified forms thereof as come withinthe scope of the following claims.

What is claimed is:
 1. A mold temperature controlling apparatus for afluid cooled mold, comprising:(a) temperature sensor means adapted to bemounted to a mold for providing an output signal indicative of thetemperature of the mold, the temperature sensor means including atemperature probe having a thermistor adapted to contact the mold, aresilient shank containing electrical leads and having a contact head atits end which houses the thermistor, and a frustum shaped sleeve ofresilient material which surrounds the shank and is adapted to be wedgedinto a well formed in a mold to thereby hold the probe firmly inposition on the mold, the temperature sensor means also includingcircuit means for providing a bias current to the thermistor and avoltage signal across the thermistor; (b) means receiving the voltagesignal across the thermistor indicative of the temperature of the moldand responsive thereto for providing a control signal when thetemperature sensed exceeds a selected control temperature; and (c) flowcontrol valve means, connected to receive the control signal and adaptedto be connected in the flow of cooling fluid to the mold, for openingand closing in response to the control signal to provide a flow ofcooling fluid to the mold when the sensed temperature exceeds theselected control temperature and to cut off the flow of cooling fluidwhen the sensed temperature drops below the selected controltemperature.
 2. A mold temperature controlling apparatus for a fluidcooled mold, comprising:(a) temperature sensor means adapted to bemounted to a mold for providing an output signal indicative of thetemperature of the mold the temperature sensor means including atemperature probe having a thermistor adapted to contact the mold,acylindrical tube having retaining rings formed on each end, an insulatedelectrical lead contained within and extending from one end of the tube,and having a contact head at its end located at the other end of thetube and which houses the thermistor, a hub having internal surfacesdefining a cylindrical slide passage adapted to slideably engage andhold the tube, a spring well formed in the hub which is co-axial withand has a larger diameter than the slide passage, a spring seat formedas a radial surface in the hub between the slide passage and the springwell walls; and exterior threads formed on the hub which are adapted tothreadingly engage a probe well in a mold which has mating threads, anda helical spring surrounding the tube and mounted in compression betweenthe spring seat in the hub and the retaining ring formed on the end ofthe tube at which the contact head is located to thereby bias the tubeand contact head away from the hub and toward firm contact with thebottom of a probe well in a mold to which the probe may be mounted, thetemperature sensor means also including circuit means for providing abias current to the thermistor and a voltage signal across thethermistor; (b) means receiving the voltage signal across the thermistorindicative of the temperature of the mold and responsive thereto forproviding a control signal when the temperature sensed exceeds aselected control temperature; and (c) flow control valve means,connected to receive the control signal and adapted to be connected inthe flow of cooling fluid to the mold, for opening and closing inresponse to the conrol signal to provide a flow of cooling fluid to themold when the sensed temperature exceeds the selected controltemperature and to cut off the flow of cooling fluid when the sensedtemperature drops below the selected control temperature.
 3. A moldtemperature controlling apparatus for a fluid cooled mold,comprising:(a) temperature sensor means adapted to be mounted to a moldfor providing an output signal indicative of the temperature of the moldthe temperature sensor means including a temperature probe having athermistor adapted to contact the mold,a casing made of magnetizedferromagnetic material having a disc shaped back and a circular sidewalldepending from the periphery of the back, and a port formed in thecasing, an insulated electrical lead extending through the port into theinterior of the casing and terminating in a contact head which containsthe thermistor, filler material filling the interior of the casing andsecuring the lead and the contact head therein, the temperature sensormeans also including circuit means for providing a bias current to thethermistor and a voltage signal across the thermistor; (b) meansreceiving the voltage signal across the thermistor indicative of thetemperature of the mold and responsive thereto for providing a controlsignal when the temperature sensed exceeds a selected controltemperature; and (c) flow control valve means, connected to receive thecontrol signal and adapted to be connected in the flow of cooling fluidto the mold, for opening and closing in response to the control signalto provide a flow of cooling fluid to the mold when the sensedtemperature exceeds the selected control temperature and to cut off theflow of cooling fluid when the sensed temperature drops below theselected control temperature.
 4. A mold temperature controllingapparatus for a fluid cooled mold, comprising:(a) temperature sensormeans adapted to be mounted to a mold for providing an output signalindicative of the temperature of the mold; (b) means responsive to thesignal indicative of the temperature of the mold for providing a controlsignal when the temperature sensed exceeds a selected controltemperature; and (c) flow control valve means, adapted to be connectedin the flow of cooling fluid to the mold and connected to receive thecontrol signal, for opening and closing in response to the controlsignal, the flow control valve means opening to provide a flow ofcooling fluid to the mold when the sensed temperature exceeds theselected control temperature and the control signal is present andclosing to cut off the flow of cooling fluid when the sensed temperaturedrops below the selected control temperature and the control signal isremoved.
 5. Improved plastic injection molding apparatus of the typehaving a mold body, walls within the mold body defining a mold cavityinto which liquid plastic material is injected, and walls within themold body defining channels through which a cooling fluid may be passed,the improvement comprising:(a) temperature sensor means, mounted to themold body in position to sense the temperature thereof, for providing anoutput signal indicative of the temperature of the mold; (b) meansresponsive to the signal indicative of the temperature of the mold forproviding a control signal when the temperature sensed exceeds aselected control temperature; and (c) flow control valve means,connected to deliver cooling fluid to the channels in the mold and alsoconnected to receive the control signal, and being adapted to beconnected to a source of cooling fluid under pressure, for opening andclosing in response to the control signal, the flow control valve meansopening to provide a flow of cooling fluid to the mold when the sensedtemperature exceeds the selected control temperature and the controlsignal is present and closing to cut off the flow of cooling fluid whenthe sensed temperature drops below the selected control temperature andthe control signal is removed.
 6. The apparatus of claim 4 or 5including means connected to receive the control signal and responsivethereto for providing a signal to the operator when the control signalis present and cooling fluid is thereby flowing to the mold.
 7. Theapparatus for claim 4 or 5 including a variable flow regulating valveconnected in the flow of cooling fluid to the mold to allow the flowrate of cooling fluid to be adjusted to a selected maximum rate.
 8. Theapparatus of claim 1 or 2 including means responsive to the signalindicative of the temperature of the mold for providing a warning signalto the operator of the mold when the temperature sensed exceeds apreselected alarm temperature which is higher than the controltemperature.
 9. The apparatus of claim 4 or 5 including means responsiveto the signal indicative of the temperature of the mold for providing awarning signal to the operator of the mold when the temperature sensedis less than a preselected under temperature level which is less thanthe control temperature.
 10. The apparatus of claim 4 or 5 wherein thetemperature sensor means includes a temperature probe having athermistor adapted to contact the mold and circuit means for providing abias current to the thermistor and for transmitting the voltage signalacross the thermistor to the means responsive to the signal indicativeof the temperature of the mold.
 11. The apparatus of claim 10 includingan under temperature comparator having two inputs, a first of the inputsbeing connected to receive the voltage signal across the thermistor,means for applying an adjustable voltage level to the second of thecomparator inputs such that the comparator output will switch stateswhen the voltage at its first input is lower than the voltage at itssecond input, whereby the voltage level applied to the second input ofthe under temperature comparator may be adjusted such that thecomparator switches states when the temperature of the thermistor isbelow a selected minimum temperature, and an under temperature warninglight connected to the output of the under temperature comparator. 12.The apparatus of claim 10 wherein the temperature probe comprises:(1) aresilient shank containing electrical leads and having a contact head atits end which houses the thermistor; (2) a frustum shaped sleeve ofresilient material which surrounds the shank and is adapted to be wedgedinto a well formed in a mold to thereby hold the probe firmly inposition on the mold.
 13. The apparatus of claim 10 wherein thetemperature probe comprises:(1) a cylindrical tube having retainingrings formed on each end; (2) an insulated electrical lead containedwithin and extending from one end of the tube, and having a contact headat its end located at the other end of the tube and which houses thethermistor; (3) a hub having internal surfaces defining a cylindricalslide passage adapted to slideably engage and hold the tube, a springwell formed in the hub which is co-axial with and has a larger diameterthan the slide passage, a spring seat formed as a radial surface in thehub between the slide passage and the spring well walls; and exteriorthreads formed on the hub which are adapted to threadingly engage aprobe well in a mold which has mating threads; and (4) a helical springsurrounding the tube and mounted in compression between the spring seatin the hub and the retaining ring formed on the end of the tube at whichthe contact head is located to thereby bias the tube and contact headaway from the hub and toward firm contact with the bottom of a probewell in a mold to which the probe may be mounted.
 14. The apparatus ofclaim 10 wherein the temperature probe comprises:(1) a casing made ofmagnetized ferromagnetic material having a disc shaped back and acircular sidewall depending from the periphery of the back, and a portformed in the casing; (2) an insulated electrical lead extending throughthe port into the interior of the casing and terminating in a contacthead which contains the thermistor; and (3) filler material filling theinterior of the casing and securing the lead and the contact headtherein.
 15. The apparatus of claim 10 wherein the means responsive tothe signal indicative of temperature includes a control comparatorhaving two inputs, a first of the inputs being connected to receive thevoltage signal across the thermistor, means for applying an adjustablevoltage level to a second of the comparator inputs such that thecomparator output will switch states when the voltage at its first inputis lower than the voltage at its second input, whereby the voltage levelapplied to the second input of the control comparator may be adjustedsuch that the comparator switches states when the temperature of thethermistor exceeds the selected maximum temperature.
 16. The apparatusof claim 15 wherein the flow control valve includes a solenoid connectedto a normally closed two-way valve connected in the cooling fluid flowpath, and further including valve driver circuit means, connected toreceive the output of the control comparator, for providing electricalpower to the valve solenoid when the output of the control comparator ishigh.
 17. The apparatus of claim 15 including an alarm comparator havingtwo inputs, a first of the inputs being connected to receive the voltagesignal across the thermistor, means for applying an adjustable voltagelevel to a second of the comparator inputs such that the comparatoroutput will switch states when the voltage at its first input is lowerthan the voltage at its second input, whereby the voltage applied to thesecond input of the alarm comparator may be adjusted such that thecomparator switches states when the temperature of the thermistorexceeds a selected alarm temperature, further including oscillatormeans, connected to receive the output signal from the alarm comparator,for providing an audio frequency signal output when the output of thealarm comparator is high, a speaker, and speaker driver circuit meansreceiving the output of the oscillator and providing a power signal atthe oscillator frequency to the speaker.
 18. The apparatus of claim 17wherein the oscillator provides an output signal which alternates at alower than audible frequency between two distinguishable audiofrequencies.
 19. The apparatus of claim 18 including an alarm warninglight connected to the oscillator to receive power to turn it on and offas the oscillator alternates between output frequencies.